1. Field of the Invention
The present invention relates to a cable with coating of a composite material.
More particularly, the present invention relates to a cable, in particular for the transmission or distribution of low-voltage or medium-voltage power, which has a coating layer consisting of a composite material that possesses high mechanical, thermal, electrical and barrier properties (very low permeability to water, to solvents and to gases) and fire resistance.
Said cable permits, in particular, the use of high operating temperatures.
2. Description of the Related Art
It is now a requirement in the power and telecommunication cables sector, to find materials that can be used for making coatings that surround the conductor that are able to improve the properties enumerated above.
For this purpose, for example, the addition of mineral fillers to the polymer matrix constituting the coatings that surround the conductor is known, said fillers imparting semiconducting properties to said coatings, for example carbon black and the like; or various types of mineral fillers that are able to impart fireproofing properties to said coatings, for example calcined kaolin, calcium carbonate, magnesium hydroxide, aluminium hydroxide and the like.
Patent application GB 2,113,453 describes an electric cable comprising one or more conductors, a semiconducting screen, a semiconducting layer, an insulating layer, a metallic screen and a sheath. A material that is able to absorb water and so to reduce xe2x80x9cwater-treeingxe2x80x9d, comprising an uncoated montmorillonite, is interposed between the semiconducting layer and the sheath. Instead of the montmorillonite it is possible to use molecular sieves as such or mixed with said montmorillonite. Preferably, said material that is able to absorb water is mixed with an organic carrier so as to form an extrudable mixture or a paste. The carrier can be a viscous organic fluid or a semisolid rubber, for example unvulcanized or depolymerized butyl rubber or polyisobutylene.
Patent application EP 1,033,724 describes an electric cable having at least one coating consisting of a material comprising an organic compound, in particular a polymer, and an inorganic material with lamellar structure. The polymer can be selected from: polyolefins (for example, polyethylene or polypropylene), polybutylene terephthalate, vinyl polymers, elastomers, which may be halogenated, thermoplastic elastomers, silicones, copolymers of ethylene, or their mixtures. Among the copolymers of ethylene, the following can be used: ethylene/vinyl acetate copolymer, ethylene/propylene copolymer, ethylene/ethyl acrylate copolymer, ethylene/methyl acrylate copolymer, ethylene/acrylic acid copolymer, terpolymers of ethylene, polymers of ethylene functionalized with groups such as, for example, acids, epoxides, etc. In some cases it is also possible to use epoxy resins, polyesters, polyamides, polyurethanes, silicones, or their mixtures. A graphite or an inorganic oxide can be used as the inorganic material with lamellar structure. Preferably, the organic oxide is a silicate, in particular an aluminosilicate, for example a natural or artificial clay. Among the clays, it is possible to use, for example, kaolin, smectite, montmorillonite, bentonite, and the like. Said inorganic compound with lamellar structure is treated with a compatibilizing agent, which can be a quaternary ammonium salt, a polyethylene oxide, a phosphorus derivative, etc.
The Applicant has found, however, that the use of the aforesaid material comprising a polymer and an inorganic material with lamellar structure does not always endow the cable with improved properties. In particular, the Applicant has found that, for the purpose of obtaining a cable that possesses improved properties, the polymer and the inorganic material with lamellar structure must be combined in a suitable manner.
The Applicant has now found that it is possible to improve said properties by making a cable that is provided with at least one coating of composite material, said composite material being obtained by intercalation of a polymer containing a predetermined quantity of polar groups, in an inorganic material with lamellar structure surface-treated with a predetermined quantity of a compatibilizer. The Applicant has found that said intercalation only occurs when using a polymer and an inorganic material with lamellar structure that are mutually compatible. More particularly, achievement of a sufficient degree of intercalation depends essentially on the correct combination of the quantity of polar groups present in the polymer and the quantity of compatibilizer present in the inorganic material with lamellar structure. Once intercalation of the polymer has occurred, with consequent exfoliation of the inorganic material with lamellar structure, a composite material is obtained in which said inorganic material is distributed homogeneously in the polymer and has a particle size of the order of a nanometre. On the other hand, if a polymer and an inorganic material with lamellar structure are selected that are not compatible, the aforesaid intercalation/exfoliation does not occur, and a simple mixture between the two components (polymer/inorganic material with lamellar structure) is thus obtained, in which mixture the particles of said inorganic material still have their initial dimensions (of the order of microns). For equal final performance of the cable, when intercalation occurs it is possible to use a smaller quantity of inorganic material with lamellar structure, with respect to the quantity used in the case when intercalation does not occur.
In a first aspect, the present invention therefore relates to a cable comprising at least one conductor and at least one coating layer consisting of a composite material comprising:
at least one polymer containing a predetermined quantity of polar groups; and
at least one inorganic material with lamellar structure, surface-treated with a predetermined quantity of a compatibilizer;
in which said predetermined quantity of polar groups and said predetermined quantity of compatibilizer are such that said inorganic material exhibits, in the composite material, a d-spacing value in X-ray diffraction analysis at least 20% higher, preferably at least 30% higher, with respect to the d-spacing value of the inorganic material as such.
In the present description and in the subsequent claims, the term xe2x80x9cconductorxe2x80x9d means a conducting element as such, of elongated shape and preferably of a metallic material, or a conducting element coated with a semiconducting layer.
In a second aspect the present invention relates to a composite material comprising:
at least one polymer containing a predetermined quantity of polar groups; and
at least one inorganic material with lamellar structure, surface-treated with a predetermined quantity of a compatibilizer;
in which said predetermined quantity of polar groups and said predetermined quantity of compatibilizer are such that said inorganic material exhibits, in the composite material, a d-spacing value in X-ray diffraction analysis at least 20% higher, preferably at least 30% higher, with respect to the d-spacing value of the inorganic material as such.
In a further aspect, the present invention relates to the use of a composite material as defined above as a base material for the preparation of a coating layer with properties of electrical insulation, or for the preparation of a coating layer with semiconducting properties, or for the preparation of a coating layer with the function of external protective sheath.
The polymer containing polar groups is generally selected from polymers functionalized with polar groups selected from: hydroxyl groups, carboxyl groups, epoxide groups, anhydride groups, silane groups.
Said polar groups can be introduced into the polymer during the production of the polymer itself, by copolymerization with corresponding functionalized monomers containing at least one ethylenic unsaturation; or by subsequent modification of the polymer by grafting-on of the aforesaid functionalized monomers in the presence of a radical initiator (in particular an organic peroxide).
Alternatively, it is possible to introduce the polar groups by reaction of groups already present on the polymer with a suitable reagent such as, for example, by an epoxidation reaction of a diene polymer containing double bonds along the main chain and/or side groups with a peracid (for example, m-chloroperbenzoic acid or peracetic acid) or with hydrogen peroxide in the presence of a carboxylic acid or a derivative of a carboxylic acid; or by a hydrolysis reaction of groups already present on the polymer, such as, for example, by a hydrolysis reaction of vinyl acetate groups.
Examples of epoxides containing an ethylenic unsaturation that can be used for this purpose are: glycidyl acrylate, glycidyl methacrylate, monodiglycidyl ester of itaconic acid, glycidyl ester of maleic acid, vinylglycidyl ether, allylglycidyl ether, or their mixtures.
Examples of monocarboxylic or dicarboxylic acids having at least one ethylenic unsaturation, or their derivatives, that can be used for this purpose are: maleic acid, maleic anhydride, fumaric acid, citraconic acid, itaconic acid, acrylic acid, methacrylic acid or their mixtures and the anhydrides or esters derived from them or their mixtures. Maleic anhydride is particularly preferred.
Examples of silane compounds that can be used for this purpose are: xcex3-methacryloxypropyltrimethoxysilane, methyltriethoxysilane, methyltris-(2-methoxyethoxy)silane, dimethyldiethoxysilane, vinyltris-(2-methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, octyltriethoxysilane, isobutyltriethoxysilane, isobutyltrimethoxysilane, or their mixtures.
Examples of polymers functionalized with the polar groups described above are: polyolefins functionalized with polar groups such as, for example, polypropylene or polyethylene grafted with maleic anhydride; ethylene-vinyl acetate (EVA) copolymers; ethylene-ethyl acrylate (EEA) copolymers; ethylene-methyl acrylate (EMA) copolymers; ethylene-acrylic acid copolymers; polymethyl methacrylate (PMMA).
Alternatively, the polymer containing polar groups can be selected from: polymers containing amide groups such as polyamides, for example nylon 6; halogenated polymers such as, for example, polyvinylchloride; elastomers functionalized with the polar groups described above such as, for example, epoxidized natural rubber (ENR), natural rubber functionalized with acrylic or methacrylic groups; halogenated elastomers such as, for example, polyethylenechlorosulphonate (CSP), chlorinated polyethylene (CPE), said halogenated elastomers optionally containing hetero atoms such as, for example, oxygen, nitrogen, sulphur; elastomeric copolyesters obtained by transesterification of terephthalic anhydride with a mixture of diols such as, for example, Hytrel(copyright) from DuPont; acrylates such as, for example, polyacrylates.
For the purpose of the present invention, the polymer containing a predetermined quantity of polar groups can be selected from polymers of low polarity having a content of hetero atoms of between 0.5% and 7% by weight with respect to the weight of the polymer; or from polymers of medium polarity having a content of hetero atoms of between 7% and 32% by weight; or from polymers of high polarity having a content of hetero atoms of between 32% and 50% by weight.
Within the framework of the present description and the claims, the term xe2x80x9chetero atomsxe2x80x9d means atoms different from carbon and hydrogen, and in particular it means atoms of oxygen, nitrogen, sulphur, chlorine or bromine.
Examples of said polymers of low polarity are: polyolefins functionalized with polar groups such as, for example, polypropylene or polyethylene grafted with maleic anhydride; ethylene-vinyl acetate (EVA) copolymers having a content of vinyl acetate between 2% and 18% by weight; ethylene-ethyl acrylate (EEA) copolymers having a content of ethyl acrylate between 2% and 18% by weight; ethylene-methyl acrylate (EMA) copolymers having a content of methyl acrylate between 2% and 21% by weight; ethylene-acrylic acid copolymers; epoxidized natural rubber (ENR), natural rubber functionalized with acrylic or methacrylic groups.
Examples of said polymers of medium polarity are: ethylene-vinyl acetate (EVA) copolymers having a content of vinyl acetate of between 18% and 80% by weight; ethylene-ethyl acrylate (EEA) copolymers having a content of ethyl acrylate of between 18% and 80% by weight; ethylene-methyl acrylate (EMA) copolymers having a content of methyl acrylate of between 21% and 80% by weight; polymethyl methacrylate (PMMA); polyesters such as, for example, polybutylene terephthalate (PBT), polyethylene terephthalate (PET); elastomeric copolyesters obtained by transesterification of terephthalic anhydride with a mixture of diols; halogenated elastomers such as, for example, polyethylene chlorosulphonate (CSP), chlorinated polyethylene (CPE), said halogenated elastomers possibly containing hetero atoms such as, for example, oxygen, nitrogen, sulphur; acrylates such as, for example polyacrylates.
Examples of said polymers of high polarity are: ethylene-vinyl acetate (EVA) copolymers having a content of vinyl acetate greater than 80% by weight; ethylene-ethyl acrylate (EEA) copolymers having a content of ethyl acrylate greater than 80% by weight; ethylene-methyl acrylate copolymers having a content of methyl acrylate greater than 80% by weight; polyvinyl acetate; polyethyl acrylate; polymethyl acrylate; polyvinyl chloride (PVC).
Examples of polymers containing polar groups that can be used in the present invention and are available commercially are: Escorene(copyright) Ultra UL00119 from Exxon Chemical; Vestodur(copyright) 300 from Hxc3xcls; Elvax(copyright) 40L03 from DuPont; Hytrel(copyright) from DuPont, Orevac(copyright) from Atofina.
The inorganic material with lamellar structure can be selected from phyllosilicates, such as: smectites, for example montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite; vermiculite; halloisite; sericite; or their mixtures. Montmorillonite is particularly preferred.
The compatibilizer can be selected from the quaternary ammonium or phosphonium salts having general formula (I): 
in which:
Y represents N or P;
R1, R2, R3 and R4, which may be identical or different, represent a linear or branched C1-C20 alkyl or hydroxyalkyl group; a linear or branched C1-C20 alkenyl or hydroxyalkenyl group; a group xe2x80x94R5xe2x80x94SH or xe2x80x94R5xe2x80x94NH in which R5 represents a linear or branched C1-C20 alkylene group; a C6-C18 aryl group; a C7-C20 arylalkyl or alkylaryl group; a C5-C18 cycloalkyl group, said cycloalkyl group possibly containing a hetero atom such as oxygen, nitrogen or sulphur;
Xnxe2x88x92 represents an anion such as the chloride ion, the sulphate ion or the phosphate ion;
n represents 1, 2 or 3.
The inorganic material with lamellar structure can be selected from the inorganic materials with lamellar structure of low polarity obtained by surface treatment with a predetermined quantity of a compatibilizer having general formula (I) in which at least two of the substituents R1, R2, R3 and R4, represent a linear or branched C4-C20 alkyl group, preferably C18, said quantity being between 125 meq and 200 meq per 100 g of inorganic material with lamellar structure.
Alternatively, the inorganic material with lamellar structure can be selected from the inorganic materials with lamellar structure of medium polarity obtained by surface treatment with a predetermined quantity of a compatibilizer having general formula (I) in which at least one of the substituents R1, R2, R3 and R4, represents a linear or branched C4-C20 alkyl or hydroxyalkyl group, or a group xe2x80x94R5xe2x80x94SH or xe2x80x94R5xe2x80x94NH in which R5 represents a linear or branched C4-C20 alkylene group, said quantity being between 95 meq and 125 meq per 100 g of inorganic material with lamellar structure.
Alternatively, the inorganic material with lamellar structure can be selected from the inorganic materials with lamellar structure of high polarity obtained by surface treatment with a predetermined quantity of a compatibilizer having general formula (I) in which at least one of the substituents R1, R2, R3 and R4, represents a linear or branched C4-C20 alkyl or hydroxyalkyl group, or a linear or branched C4-C20 alkenyl or hydroxyalkenyl group, or a group xe2x80x94R5xe2x80x94SH or xe2x80x94R5xe2x80x94NH in which R5 represents a linear or branched C4-C20 alkylene group, said quantity being between 20 meq and 95 meq per 100 g of inorganic material with lamellar structure.
The surface treatment of the inorganic material with lamellar structure with the compatibilizer can be carried out according to known methods such as, for example, by an ion exchange reaction between the inorganic material with lamellar structure and the compatibilizer: further details are described, for example, in U.S. Pat. No. 4,136,103, U.S. 5,747,560 or U.S. 5,952,093.
According to a preferred embodiment, the composite material comprises:
at least one polymer of low polarity having a content of hetero atoms between 0.5% and 7% by weight; and
at least one inorganic material with lamellar structure of low polarity obtained by surface treatment with a predetermined quantity of a compatibilizer having general formula (I) in which at least two of the substituents R1, R2, R3 and R4, represent a linear or branched C4-C20 alkyl group, preferably C18, said quantity being between 125 meq and 200 meq per 100 g of inorganic material with lamellar structure.
According to another preferred embodiment, the composite material comprises:
at least one polymer of medium polarity having a content of hetero atoms between 7% and 32% by weight; and
at least one inorganic material with lamellar structure of medium polarity obtained by surface treatment with a predetermined quantity of a compatibilizer having general formula (I) in which at least one of the substituents R1, R2, R3 and R4, represents a linear or branched C4-C20 alkyl or hydroxyalkyl group, or a group xe2x80x94R5xe2x80x94SH or xe2x80x94R5xe2x80x94NH in which R5 represents a linear or branched C4-C20 alkylene group, said quantity being between 95 meq and 125 meq per 100 g of inorganic material with lamellar structure.
According to a further preferred embodiment, the composite material comprises:
at least one polymer of high polarity having a content of hetero atoms between 32% and 50% by weight; and
at least one inorganic material with lamellar structure of high polarity obtained by surface treatment with a predetermined quantity of a compatibilizer having general formula (I) in which at least one of the substituents R1, R2, R3 and R4, represents a linear or branched C4-C20 alkyl or hydroxyalkyl group, or a linear or branched C4-C20 alkenyl or hydroxyalkenyl group, or a group xe2x80x94R5xe2x80x94SH or xe2x80x94R5xe2x80x94NH in which R5 represents a linear or branched C4-C20 alkylene group, said quantity being between 20 meq and 95 meq per 100 g of inorganic material with lamellar structure.
Examples of inorganic materials with lamellar structure that can be used in the present invention and are available commercially are the products known by the name Cloisite(copyright) from Southern Clay Products.
The inorganic material with lamellar structure is added in a quantity between 1 phr and 40 phr, preferably between 4 phr and 20 phr.
Within the present description and the claims that follow, the term xe2x80x9cphrxe2x80x9d indicates parts by weight of a given ingredient per 100 parts of polymer.
The composite material of the present invention can be produced in accordance with methods known in the art, as described for example in U.S. Pat. No. 5,747,560 or U.S. 5,952,093.
For example, said composite material can be produced by a method comprising the following stages:
heat the polymer containing polar groups to a temperature not below the softening point of said polymer, preferably not below the melting temperature of said polymer;
mix the polymer with said inorganic material with lamellar structure, and bring the mixture up to a temperature not below the softening point of said polymer, preferably not below the melting temperature of said polymer, applying a force sufficient to disaggregate, at least partially, said inorganic material with lamellar structure.
The composite material according to the present invention can be prepared by melt blending, using an extruder, an internal mixer of the type with tangential rotors (Banbury), or in continuous mixers of the Ko-Kneader type (Buss), or co-rotating or counter-rotating twin-screw types (Brabender).
For the production of a coating layer for a cable, other conventional components can be added to the composite material as defined above such as, for example, antioxidants, processing adjuvants, water-tree retardants.
Conventional antioxidants suitable for this purpose are, for example, distearylthiodipropionate, tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxymethyl]methane, or their mixtures.
Processing adjuvants that can be added to the composite material are, for example, calcium stearate, zinc stearate, stearic acid, paraffin wax, or their mixtures.
With particular reference to low- and medium-voltage cables, the composite material as defined above can be used advantageously for making a coating layer with properties of electrical insulation. In fact, as described above, said composite material can give high mechanical characteristics both at room temperature and when hot, in particular permitting the use of high operating temperatures.
Coating layers with semiconducting properties of the cable can be produced in accordance with the known art and consist advantageously of a semiconducting polymeric material. Preferably, said material is of the same type as that used for the coating layer with electrical insulation properties, so as to ensure good adhesion and hence avoid detachments that would generate partial discharges and, ultimately, perforation of the cable.
According to a preferred aspect, at least one of the coating layers with semiconducting properties of the cable of the invention comprises a composite material as described above.
In the case when it is intended to make a semiconducting layer, in general a conducting filler is dispersed in the polymeric material, in particular carbon black, in a quantity such as to endow said material with semiconducting characteristics (i.e. so as to obtain a resistivity of less than 5 xcexa9.m at room temperature). Said quantity is generally between 5% and 80% by weight, preferably between 10% and 50% by weight, with respect to the total weight of the final composition.
According to another preferred aspect, the present invention relates to a cable comprising, in addition to the layers defined above, at least one coating layer with the function of external protective sheath, consisting of the composite material described above.
In accordance with the present invention, the use of the composite material described above makes it possible to obtain cables possessing high mechanical, thermal, electrical and barrier properties (very low permeability to water, solvents and gases) and fire resistance.
In particular, the coating layer with electrical insulation properties, produced using the composite material described above, makes it possible to have relatively high operating temperatures (even 15xc2x0 C. higher with respect to the conventional materials, for example XLPE). In this way, the mechanical strains are avoided, to which the known materials are generally liable, for example XLPE, which generally constitute the coating layer with electrical insulation properties, when using high operating temperatures.
Within the scope of the present invention, xe2x80x9clow voltagexe2x80x9d generally means a voltage up to 1 kV, whereas xe2x80x9cmedium voltagexe2x80x9d means a voltage between 1 kV and 35 kV.
Although the present description mainly focuses on the production of cables for the transmission or distribution of low- or medium-voltage electric power, the composite material described above can be used for coating electrical devices in general, and in particular various types of cables, for example high-voltage cables, cables for telecommunications, or even mixed power/telecommunication cables.